schema for Azure Policy artifacts

Would you consider adding the schema for Azure Policy artifacts (or is there a canonical place I can get it from)

EPAC's version looks pretty close, if we remove their additions

https://github.com/Azure/enterprise-azure-policy-as-code/tree/main/Schemas

It feels like it should be either in this library or in https://github.com/Azure/azure-policy but I suspect the latter will take more effort :)

Start by reviewing the schemas in the linked EPAC Schemas directory and comparing them with the Azure Policy artifacts this library currently supports. Clarify whether the expected outcome is adding the schema here or identifying a canonical external source; done should include an agreed location and complete Azure Policy artifact schema coverage.

Written by the indexing model from the issue text.